BRU is an international airport operated the clock round, it's open 24/7/365. Even some shops are open the clock round

In general, UA's checkin opens 3h before flight. So it should be open at 3h15 but since it's only a intra EU flight chances excist that the checkin will only be open at 4h15. However, don't expect to visit the lounge, SN's lounge will only open at 5h30.